1. Field of the Invention
The present invention relates generally to the field of high density multi-dimensional nonvolatile memory arrays and particularly to multi-dimensional memory arrays made of frequency-addressable spin transfer torque (STT) memory elements, each including at least one free layer, wherein each memory element has a different resonant frequency, due to the shape and material of the memory element, thereby creating large nonvolatile memory arrays.
2. Description of the Prior Art
High density nonvolatile memory devices, based on flash technology, have become increasingly popular for use in many and diverse applications, computing being one of them.
However, such technology is approaching practical limits for higher areal densities due to lithographic constraints. The critical lithographic dimension F is currently around 45 nanometers (nm) for flash technology, and is projected to decrease by around 20% per annum in the near future, although reducing the size of F beyond factor of approximately two will be very difficult. The corresponding bit size is approaching 4 F2 for single-bit-per-cell flash memory, and 2 F2 for double-bit-per-cell flash memory. While multiple bits may in principle be stored in a single cell, which increases areal density, this has proved impractical beyond 2 bits/cell because signal-to-noise ratios are reduced due the presence of multiple, closely spaced, levels in one memory cell.
Alternative storage devices, comprising single-bit-per-cell multi-layer arrangements of storage elements, have been demonstrated, for example by Matrix Semiconductor, Inc. of Santa Clara, Calif. To date, such multi-layer storage devices have allowed only write-once or one-time-write operation, and have not allowed multiple write operations to memory. New materials for re-writable memory are a topic of current research and require new inventions to be integrated into multilayer devices with large storage capacities.
One class of solid state memory devices, or nonvolatile memory, is Magnetic Random Access Memory (MRAM). MRAM devices are based on magnetic materials. MRAM devices comprise cells or elements having a magnetically hard layer (the “pinned” or “fixed” layer) and a magnetically soft layer (the “free” layer). Writing to MRAM is performed by passing current through current leads that are formed on either side of each memory element in order to create a local induced magnetic field which sets the direction of the soft layer magnetization. Significant problems have been encountered however in scaling these devices to high densities. In particular, disturbances to neighboring cells or elements can occur during writing, sometimes causing a neighboring cell to be erroneously written.
Spin Transfer Torque (STT) devices are similar to MRAM devices except that the current paths pass through the magnetic layers of each memory element, rather than to the side of each memory element, and the soft layer of the memory element is set via the transfer of spin torque from the spin polarized current passing through that layer. However, this approach requires rather high current densities, which are undesirable due to heat and power consumption concerns. In addition, this approach is difficult to scale to high areal densities using a multilevel architecture approach, as this would require cells with multiple free layers and it would be generally difficult to switch each layer independently with only a spin polarized direct current.
In light of the foregoing, there is a need for a high density three-dimensional nonvolatile memory array, which incorporates multiple layers of memory elements, where each memory element can be switched independently.